The present invention relates to microelectronic packages having leads or traces and specifically relates to microelectronic packages having arrays of resilient leads and methods of making such microelectronic packages.
Complex microelectronic devices such as semiconductor chips typically require numerous connections to other electronic components. For example, a complex device including a semiconductor chip may require hundreds of electrical connections between the chip and one or more external devices. These electrical connections may be made using several alternative methods, including wire bonding, tape automated bonding and flip-chip bonding. Each of these techniques presents various problems including difficulty in testing the chip after bonding, long lead lengths, large areas occupied by the chip on a microelectronic assembly, and fatigue of the connections due to changes in size of the chip and the substrate during thermal expansion and contraction.
In many microelectronic devices, it is desirable to provide an electrical connection between components that can accommodate relative movement between the components. For example, where a semiconductor chip is mounted to a circuit board, thermal expansion and contraction of the chip and circuit board can cause the contacts on the chip to move relative to contacts on the circuit board. This movement can occur during operation of the device and can also occur during manufacturing operations (e.g. when soldering the chip to the circuit board).
One structure that has been used to successfully address these problems is commonly referred to as an xe2x80x9cinterposerxe2x80x9d or xe2x80x9cchip carrierxe2x80x9d, such as that shown in certain preferred embodiments of commonly assigned U.S. Pat. Nos. 5,148,265, 5,148,266 and 5,455,390, the disclosures of which are hereby incorporated by reference herein. Interposers typically include a flexible, sheet-like element having a plurality of terminals disposed thereon, and including flexible leads used to connect the terminals with contacts on a microelectronic element, such as a semiconductor chip or wafer. The flexible leads permit thermal expansion of the various components without inducing stresses in the connection. The terminals of the interposer may then be used to test the assembly, and/or permanently attach the assembly to another microelectronic element.
A compliant layer may be disposed between a microelectronic element and the interposer. The compliant layer typically encapsulates the leads connecting the interposer and microelectronic element and facilitates connection of the terminals to a test device and/or to the final electronic assembly by compensating for variations in component flatness and terminal heights.
As illustrated in certain preferred embodiments of commonly assigned U.S. Pat. No. 5,518,964 (xe2x80x9cthe ""964 patentxe2x80x9d), the disclosure of which is hereby incorporated by reference herein, an array of moveable electrical connections between two microelectronic elements, such as a semiconductor chip and a substrate, can be provided by first connecting leads between the microelectronic elements and then moving the elements away from one another through a predetermined displacement so as to bend the leads. One of the microelectronic elements may be a connection component including a dielectric body having leads extending along a surface of the dielectric body. The leads may have first ends permanently attached to the dielectric body and second ends releasably attached to the dielectric body. The dielectric body, with the leads thereon, may be juxtaposed with a semiconductor chip having contacts and the second releasable ends of the leads may be bonded to the contacts on the chip. Following bonding, the dielectric body and chip are moved away from one another, thereby bending the leads toward a vertically extensive disposition. During or after movement, a curable material such as a liquid composition may be introduced between the elements. The curable material may then be cured, such as by using heat, to form a compliant dielectric layer surrounding the leads. The resulting semiconductor chip package has terminals on the dielectric body or connection component which are electrically connected to the contacts on the chip, but which can move relative to the chip so as to compensate for thermal effects. For example, the semiconductor chip package may be mounted to a circuit board by solder-bonding the terminals to conductive pads on the circuit board. Relative movement between the circuit board and the chip due to thermal effects is allowed by the moveable interconnection provided by the leads and the compliant layer.
In other embodiments of the ""964 patent, the package-forming process can be conducted on a wafer scale, so that all of the semiconductor chips in a wafer may be connected to connection components in a single step. The resulting wafer package is then severed so as to provide individual units, each including one or more of the chips and a portion of the dielectric body. The above-described leads may be formed on the chip or wafer, rather than on the dielectric body. In further embodiments of the ""964 patent, a dielectric body having terminals and leads is connected to terminal structures on a temporary sheet. The temporary sheet and dielectric body are moved away from one another so as to vertically extend the leads, and a curable liquid material is introduced around the leads and cured so as to form a compliant layer between the temporary sheet and the dielectric body. The temporary sheet is then removed, leaving the tip ends of the terminal structures projecting from a surface of the compliant layer. Such a component, commonly referred to as a connection component, may be used between two other components. For example, the terminal structures may be engaged with a semiconductor chip and the terminals engaged with a circuit panel or other microelectronic component.
In certain preferred embodiments of commonly assigned U.S. Pat. No. 6,117,694, the disclosure of which is hereby incorporated herein by reference, a microelectronic component, such as a connector or a packaged semiconductor device, is made by connecting multiple leads between a pair of elements and moving the elements away from one another so as to bend the leads toward a vertically extensive disposition. One of the elements may include a temporary support that may be removed after bending the leads
After the leads interconnect the microelectronic elements, an encapsulant, such as a flowable, curable dielectric material, may be injected between the microelectronic elements. The encapsulant may be injected between the microelectronic elements immediately after bonding, whereby the force of the pressurized encapsulant acting on the elements separates them and bends the leads, forming a compliant lead configuration. Alternatively, the leads may be formed before injecting the encapsulant by retaining the elements against moveable platens by vacuum, and moving the platens with respect to each other, bending and forming the leads. The encapsulant is then injected while the dielectric sheet and the wafer are in their displaced positions.
After the flowable, curable dielectric material has been cured, the microelectronic assembly may be removed from the fixture, trimmed and tested. The fixture may then be reused to perform the above operations on the next microelectronic assembly.
Despite these and other advances in the art, still further improvements would be desirable.
In accordance with certain preferred embodiments of the present invention, a method of making a microelectronic package having an array of resilient leads includes providing a first element having a plurality of conductive leads on a first surface thereof. The conductive leads preferably have terminals ends permanently attached to the first element and tip ends remote from the terminal ends, the tip ends of the conductive leads being movable relative to the terminal ends. The method preferably includes providing a second element having a plurality of contacts on a first surface thereof and juxtaposing the first surface of the second element with the first surface of the first element. The tip ends of the conductive leads may then be connected with the contacts of the second microelectronic element. The first and second microelectronic elements may then be moved away from one another so as to vertically extend the conductive lead between the first and second microelectronic elements. After the moving step, a layer of a spring-like material may then be formed over the conductive leads. The layer of a spring-like material preferably has greater yield strength than the conductive leads. Although the present invention is not limited by any particular theory of operation, it is believed that providing a layer of a spring-like material having greater yield strength then the yield strength of the conductive leads produces a highly resilient composite lead able to withstand substantial flexing and bending. As used herein, the term xe2x80x9ccomposite leadxe2x80x9d means a conductive lead or trace having a core made of a first conductive material that is coated by a shell of a second conductive material.
The conductive leads may be made of a material selected from the group consisting of aluminum, gold, copper, tin, and their alloys and combinations thereof. The layer of a spring-like material formed over the conductive leads is preferably selected from the group consisting of nickel, copper, cobalt, iron, gold, silver, platinum, noble metals, semi-noble metals, tungsten, molybdenum, tin, leads, bismuth, indium, their alloys, and combinations thereof.
In certain preferred embodiments, the method includes depositing a curable liquid encapsulant between the first and second microelectronic elements and around the vertically extended composite leads. In preferred embodiments, the curable liquid encapsulant is selected from the group consisting of materials that are curable to elastomers and adhesives. The preferred elastomers and adhesives are selected from the group consisting of silicones and epoxies. In highly preferred embodiments, the curable liquid encapsulant is a composition which is curable to a silicone elastomer. After the curable liquid encapsulant is deposited, the encapsulant may be cured to provide a compliant layer between the first and second microelectronic elements and around the vertically extended leads. In preferred embodiments, the terminals are accessible at the second surface of the second microelectronic element. Conductive elements, such as solder balls, may then be attached to the terminals ends of the leads. The conductive elements may be fusible masses of conductive metal, such as tin/lead solder balls.
In preferred embodiments, the first element and/or the second element is a microelectronic element. In preferred embodiments, the microelectronic elements are selected from the group consisting of a semiconductor chips, semiconductor wafers, packaged semiconductor chips or wafers, dielectric sheets, flexible substrates, flexible circuitized substrates, printed circuit boards, and sacrificial layers. In more preferred embodiments, the first and second microelectronic elements are selected from the group consisting of semiconductor chips, semiconductor wafers, and flexible substrates. In particularly preferred embodiments, the first microelectronic element is a chip and the second microelectronic element is a flexible substrate. In certain embodiments, at least one of the microelectronic elements may be a sacrificial layer. The sacrificial layer may be removed during one step of an assembly process so as to expose either the terminal ends or the tip ends of the conductive leads. In an alternative embodiment, the sacrificial layer may be conductive and the terminal ends or the tips ends of the conductive leads may be formed and/or exposed by removing a portion of the conductive sacrificial layer.
In other embodiments, a method of making a microelectronic package having a plurality of resilient leads includes providing a first microelectronic element having conductive leads extending along a first surface thereof, the conductive leads having terminal ends permanently attached to the first microelectronic element and tip ends releasably secured to the first microelectronic element. A second microelectronic element having contacts on a first surface thereof, may then be juxtaposed with the first surface of the first microelectronic element and the tip ends of the conductive leads may be connected with the contacts of the second microelectronic element. The first and second microelectronic elements may then be moved away from one another so as to vertically extend the conductive leads between the first and second microelectronic elements. In certain preferred embodiments, the tip ends of the conductive leads may be releasably secured to the first microelectronic element. After the moving step, a layer of a spring-like material may be formed over the conductive leads. The layer of a spring-like material may be formed by plating a conductive metal over the conductive leads. The conductive metal plated over the conductive leads preferably has a higher yield strength than the conductive leads. A curable liquid encapsulant may then be disposed between the first and second microelectronic elements and around the conductive leads. The curable liquid encapsulant may be cured to form a compliant layer between the microelectronic elements and around the leads.
In still other preferred embodiments of the present invention, a method of making a microelectronic package includes providing a first microelectronic element having a first surface with a plurality of conductive leads formed thereon, each lead having a first end permanently attached to the first microelectronic element and a second end movable away from the first microelectronic element. A second microelectronic element having conductive pads accessible at a first surface thereof may then be provided and the first surface of the first microelectronic element may be juxtaposed with the first surface of the second microelectronic element. The method may also include attaching the second ends of the conductive leads with the conductive pads or contacts of the second microelectronic element. After the attaching step, the first and second microelectronic elements may be moved away from one another so as to vertically extend the conductive leads. After the moving step, a layer of a conductive metal may be formed over the conductive leads, the layer of a conductive metal having a greater yield strength then the conductive leads.
In certain preferred embodiments, the first microelectronic element is a flexible substrate, such as a flexible dielectric sheet, and the second microelectronic element is a semiconductor chip or a semiconductor wafer. In other preferred embodiments, the first microelectronic element is a semiconductor chip or wafer and the second microelectronic element is a flexible substrate such as a flexible dielectric sheet. If the first microelectronic element is a wafer, the method may also include severing the semiconductor wafer and the flexible dielectric sheet to provide a plurality of semiconductor packages, whereby each semiconductor package includes at least one semiconductor chip connected to a portion of the flexible dielectric sheet.
In yet other preferred embodiments of the present invention, a method of making semiconductor packages having resilient leads includes providing a semiconductor chip or wafer having a plurality of contacts on a first surface thereof, and providing a flexible dielectric sheet having a plurality of conductive leads over a first surface thereof, whereby each lead has a terminal end permanently attached to the flexible dielectric sheet and a tip end movable away from the first surface of the dielectric sheet. The tip ends of the leads may then be electrically interconnected with the contacts of the wafer. If a wafer is used, then the wafer and the dielectric sheet may then be moved away from one another in a controlled fashion so as to vertically extend the leads. After the wafer and dielectric sheet are moved away from one another, a layer of a spring-like material may be formed over the outer surface of the conductive leads to form composite leads. The spring-like material desirable has a greater yield strength than the yield strength of the conductive leads. A layer of a compliant material may then be disposed between the wafer and the dielectric sheet and around the composite leads. The wafer and the dielectric sheet may then be severed so as to provide a plurality of semiconductor packages, each semiconductor package including at least one semiconductor chip and a portion of the dielectric sheet.
In further preferred embodiments of the present invention, a method of making a microelectronic element includes providing a dielectric sheet having a plurality of conductive leads overlying the first surface of the sheet and a plurality of terminals accessible at a second surface of the sheet, whereby each lead has a first end permanently attached to one of the terminals and a second end movable away from the first surface of the dielectric sheet. The method includes providing a fixture having a first surface and a plurality of contacts accessible at the first surface of the fixture, juxtaposing the first surface of the fixture with the first surface of the dielectric sheet and attaching the second ends of leads with the contacts of the fixture. After the attaching step, the fixture and the dielectric sheet may be moved away from one another so as to vertically extend the leads. A layer of a conductive spring-like material may then be formed over the exterior surface of the conductive leads to form composite leads, and a layer of a curable liquid encapsulant may be provided between the fixture and the dielectric sheet and around the composite leads. The encapsulant may then be cured to form a compliant layer. After the curing step, the fixture may be removed so as to expose the contacts at a top surface of the package.
In yet other preferred embodiments of the present invention, a connection component includes a flexible substrate having a top surface and a bottom surface, and a layer of a compliant dielectric material overlying the top surface of the substrate, the compliant material having a top surface remote from the substrate. The connection component includes an array of flexible, conductive leads having first ends attached to terminals accessible at the second surface of the said substrate and second ends adjacent the top surface of the compliant layer. Each lead preferably includes a core made of a first conductive material that is surrounded by a layer of a second conductive material. Such leads are similar to the composite leads described above. The second conductive material of the lead preferably has a greater yield strength than the first conductive material. The second ends of the leads may be accessible at the tope surface of the compliant layer. The connection component may also include contacts attached to the second ends of the leads, the contacts being accessible at the top surface of the compliant layer.
These and other preferred embodiments of the present invention will be set forth in further detail below.